Inspection systems are widely used to provide security, such as at airports or other facilities, wherever it is desired to create a secured area. Generally, one or more inspection systems are established at a checkpoint. Items passing the checkpoint are inspected to determine whether a weapon, explosive or other contraband is concealed within the item.
It has long been known that penetrating radiation (such as x-rays) may be used to characterize the contents of parcels, luggage, etc. The term “x-rays” refers to electromagnetic radiation of a very short wavelength that is capable of penetrating many objects. An x-ray “beam” may be formed by a device called a “collimator,” which effectively absorbs all x-rays except those traveling in the desired beam direction. For example, if an x-ray source produces x-rays that are directed generally toward a collimator having a slit, the x-rays that hit the surface of the collimator will be absorbed, while the x-rays that pass through the slit will form a beam in the shape of a fan, commonly called a “fan beam.”
The contents of an item may be characterized by placing an array of x-ray “detectors” on the opposite side of the item from the x-ray source and collimator, and causing the beam of x-rays to pass through the item before impinging upon the detectors. The detector array may, for example, include a planar array of hundreds or thousands of discrete detectors that are intercepted by a cone-shaped x-ray beam, or, as is more common in baggage inspection systems, may include a linear array of detectors that are intercepted by a collimated fan beam. Each detector in an array generates an electronic signal having a magnitude that corresponds to the intensity of the x-rays that impacted it during a “sample interval.” Because higher-density materials in the item being scanned will absorb more x-rays than lower-density materials, the signal output by the detectors that are in the “shadow” of higher-density materials will be lower in value than the signal output by those detectors that are intercepted by x-rays that pass only through lower-density materials.
By using a conveyor, e.g., a conveyor belt or a set of rollers, to move an item though the plane of a fan beam, a series of “lines” of x-ray transmission data may be accumulated by a linear array of detectors intercepted by the beam. Each such line of data would represent a sample interval, for the entire array of detectors, taken when the item on the conveyor was at a particular position with respect to the fan beam/detector array. Using these lines of data, an image (i.e., a collection of data that represents the item under inspection) may be generated having a resolution that depends upon the number of detectors in the array, as well as the number of lines of data that were accumulated. As a practical matter, the number of data points, or “pixels,” in such an image will be limited by the number of detectors in the array multiplied by the accumulated number of lines of data.
The data points included in an image can represent any of a number of parameters. In some systems, the data points simply represent the intensity values that are measured by the respective detectors. In other systems, the data points represent attenuation measurements that are calculated, for example, by taking the inverse natural logarithm of the ratio of the radiation intensity measured by the detectors to the intensity of the incident radiation. In yet other systems, the data points represent linear density measurements that are determined based upon the calculated attenuation measurements in addition to other known parameters, such as the distance between the source and detectors, according to well-known equations and techniques. In still other systems in which the thickness of the item under inspection can somehow be measured or approximated in the direction of the rays that intersect the item under inspection, the volumetric density of corresponding sections of the item under inspection can also be calculated and used to form data points in an image.
Conventional x-ray scanners frequently determine a linear density at numerous points throughout an item under inspection. Because objects that may be inside the item under inspection frequently have recognizable density profiles, a density image formed with the x-ray scanner can provide useful information about objects inside the item under inspection. In some inspection systems, the density image is presented visually to a human operator. In other systems, computerized systems are used to automatically process the image to identify a density profile that is characteristic of a contraband object.
Images formed by many inspection systems are these types of two-dimensional projection images. Because attenuation of the radiation is related to the density of the material through which the radiation passed, making x-ray projection images in this fashion is useful to detect many types of contraband. For example, rays of radiation passing through a gun, knife or other relatively dense object will be highly attenuated. Each pixel in the image formed by measuring rays passing through such an object will appear very different from other pixels in the image. More generally, contraband objects are likely to appear in the image as a group of pixels having an attenuation different than that of other surrounding pixels. The group will form a region with an outline conforming to the silhouette of the object. Such a group of pixels may be identified as a “suspicious region” based on manual or automated processing if it has a shape and size that matches a contraband item. Densities, or other measured material properties, of the pixels in the group also may be used in the processing to identify suspicious regions.
Inspection systems are not limited to forming images based on density. Any measurable material property may be used to form an image instead of, or in addition to, density. For example, multienergy x-ray inspection systems may measure an effective atomic number, or “Zeff,” of regions within an item under inspection and may form images based on the effective atomic number measurements. In a dual energy system, for instance, detector samples may be taken for x-rays at each of two discrete energy levels, and an analysis may be performed on the accumulated data to identify the effective atomic number of the portion of the item that was intercepted by the x-rays during the sample interval. This is possible because it is known that the ratio of the intensities of the samples at the two energies is indicative of the effective atomic number.
If no suspicious region is detected in an image of an item under inspection, the item may be “cleared” and allowed to pass the checkpoint. However, if a suspicious region is found in the image, the item may be “alarmed.” Processing of an item in response to an alarm may depend on the purpose of the inspection. For example, an alarmed item may be inspected further, destroyed, blocked from passing the checkpoint, or processed in any other suitable way.
Projection imaging is well suited for finding objects that are dense enough and large enough to produce a group of pixels having a recognizable outline regardless of the orientation of the object within the item under inspection. However, projection images are not well suited for reliably detecting objects that have at least one relatively thin dimension. If the thin dimension is parallel to the rays of radiation passing through the item under inspection, the thin object, even if substantially more dense than other objects in the item under inspection will provide little overall attenuation to the rays passing through the item under inspection. Accordingly, there will be no group of pixels in the image that has an attenuation significantly different from other pixels in the image that can be recognized as a suspicious region.
To provide more accurate detection of relatively thin items, some inspection systems are constructed using computed tomography (CT). In a CT scanner, attenuation through an item under inspection is measured from multiple different directions. Frequently, these measurements are made by placing the x-ray source and detectors on a rotating gantry. An item under inspection passes through an opening in the center of the gantry. As the gantry rotates around the item, measurements are made on rays of radiation passing through the item from many different directions. These measurements can be used to compute the volumetric density, or other material property, of the item under inspection at multiple points throughout a plane through which the rays pass. Such a process is commonly called “CT reconstruction.” Each of these computed volumetric densities represents one data value of the image, frequently called a “voxel,” in a slice through the item. By moving the item under inspection through the opening in the gantry and collecting image data at multiple locations, voxels having values representative of multiple slices through the item may be collected. The voxels can be assembled into a three dimensional, or volumetric, image of the item under inspection. Even relatively thin objects may form a recognizable group of voxels in such a volumetric image.
In constructing an inspection system, projection imaging is desirable because projection images may be formed quickly and inexpensively with relatively simply equipment. CT imaging is also desirable because some objects, such as relatively thin objects, are more reliably detected in volumetric images formed by a CT scanner. However, conventional CT scanners are frequently more expensive and slower than projection scanners. Also, because a conventional CT scanner has more moving parts, it requires more frequent maintenance than a projection imaging system.
Attributes of both a projection imaging system and a CT imaging system may be combined. One example is the MVT™ imaging system sold by L-3 Communications Security and Detection Systems, Inc., of Woburn, Mass. The MVT™ system employs multiple source-detector pairs. Each pair is positioned to form a projection image of an item under inspection from a different angle than the others. The image data gathered by each of the source-detector pairs is analyzed to detect suspicious regions in the image representative of suspicious objects. Data from each image is also used together with data from the other images to facilitate such detection. For instance, data from one image that reveals the thickness of an item can be used in conjunction with linear density measurements reflected in another image to ascertain average volumetric density measurements.
The MVT™ system, like other projection scanners, provides an advantage over a CT system of not requiring a moving source and detectors. Like a CT system, it provides an advantage over a projection imaging system of being able to detect many thin objects. Though a contraband object may have a thin dimension parallel to the rays used to form one of the projection images, a contraband object having any significant size cannot have thin dimensions parallel to the rays used to form all of the x-ray projections. Accordingly, even though a contraband object may not be readily recognizable from one of the projection images formed by the MVT™ system, such a contraband item is likely to be recognizable from at least one of the other projection images.
Nonetheless, it would be desirable to improve the images formed in a system like the MVT™.